Cirith UngolCirith Ungol - The "Spider's Cleft", formerly known as Cirith Dúath.  The pass over Ephel Dúath above Minas Morgul.  The pass was protected within Mordor by a great stronghold known as the Tower of Cirith Ungol.

 

"In that dreadful light Sam stood aghast, for now, looking to his left, he could see the Tower of Cirith Ungol in all its strength.  The horn that had been seen from the other side was only its topmost turret.  Its eastern face stood up in three great tiers from a shelf in the mountain-wall below; its back was to the great cliff behind, from which it jutted out in pointed bastions, one above the other, diminishing as they rose, with sheer sides of cunning masonry that looked north-east and south-east.  About the lowest tier, two hundred feet below where Sam now stood, there was a battlemented wall enclosing a narrow court.  Its gate, upon the near south-eastern side, opened on a broad road, the outer parapet of which ran upon the brink of a precipice, until it turned southward and went winding down into the darkness to join the road that came over the Morgul Pass.  Then on it went through a jagged rift in the Morgai out into the valley of Gorgoroth and away to Barad-dûr...As he gazed at it suddenly Sam understood, almost with a shock, that this stronghold had been built not to keep enemies out of Mordor, but to keep them in.  It was indeed one of the works of Gondor long ago, an eastern outpost of the defences of Ithilien, made when, after the Last Alliance, Men of Westernese kept watch on the evil land of Sauron where his creatures still lurked." ~The Return of the King, Book VI, chap. 1 (The Tower of Cirith Ungol)